The present invention relates to an optical system to be employed in an optical disk drive to read/write data from/to an optical disk.
Recently, technology in the field of magneto-optical disk drives has been greatly improved such that a data recording density on a magneto-optical disk has reached in excess of 10 Gbits/inch2.
In such an optical disk drive, an objective optical system is mounted on an arm which is movable in a transverse direction of tracks formed on an optical disk. Firstly, the rough tracking is performed by moving the arm so that a beam spot is positioned in the vicinity of the track. Then, a fine tracking is executed by changing an incident angle of a beam incident on the objective optical system so that a position of a beam spot is adjusted, with use of a galvano mirror or the like. During the fine tracking operation, the beam spot is accurately located on one of the tracks whose pitch is, for example, 0.34 xcexcm. When the beam spot is moved on the disk surface, i.e., when the incident angle of the beam incident on the objective optical system is being changed, it is preferable that an intensity distribution of the beam does not vary. In other words, during the fine tracking, it is preferable that a coupling efficiency between the objective optical system and a light source applicable to the optical disk drive does not vary.
It is therefore an object of the invention to provide an improved optical system, which is applicable to an optical disk drive having a rotatable deflection system, and allows a relatively great degree of freedom in designing the rotatable deflection system.
For the above object, according to an aspect of the invention, there is provided an optical system for an optical disk drive for reading/writing data from/in an optical disk, the optical system comprising: a light source emitting a parallel laser beam; a deflector having a rotatable mirror surface, the parallel laser beam emitted by the light source being incident on the mirror surface and is deflected; an objective lens system facing the optical disk; and a relay lens system provided between the deflector and the objective lens system such that the mirror surface and a front principal plane of the objective lens system have a substantially conjugate relationship.
Since the relay lens system is arranged between the deflector, or a galvano mirror and the objective lens, and the reflection surface of the galvano mirror and the front principal plane of the objective lens have the conjugate relationship, even if the galvano mirror is rotated, it is ensured that the light beam reflected by the galvano mirror is incident substantially on a same portion of the objective lens, incident angle being changed depending on the rotation angle of the galvano mirror.
In one example, the relay lens system may include a single positive lens group having a positive power, the single positive lens group making a vicinity of the mirror and the front principal plane of the objective lens to have in the substantially conjugate relationship.
In particular, the light source may include a laser diode which emits a diverging laser beam having an elliptical cross section; a collimator lens which converts said diverging laser beam emitted by said laser diode into a parallel beam having an elliptical cross section; and a beam shaping prism which converts said parallel beam having said elliptical cross section into a parallel beam having a circular cross section.
Optionally, the single positive lens group may consist of a single positive lens.
Alternatively, the single positive lens group may have a plurality of lenses arranged such that at least a chromatic aberration of the objective lens is compensated.
In this case, the single positive lens group may have a single cemented lens which are composed of the plurality of lenses.
Alternatively, the single positive lens group may have a telephoto type lens group.
In another example, the relay lens system may have a first and second relay lens groups, the first and second relay lens groups being arranged such that a front focal point of the first relay lens group is located on a vicinity of a central portion of the mirror surface on which the laser beam is incident, that a rear focal point of the first relay lens group coincides with a front focal point of the second relay lens group, and that a rear focal point of the second relay lens group coincides with a front principal point of the objective lens.
In this case, a focal length of the first relay lens group may be equal to a focal length of the second relay lens group. Alternatively, a focal length of the first relay lens group is greater, or smaller than a focal length of the second relay lens group.
Optionally, at least one of the first and second relay lens groups may be constituted to compensate a chromatic aberration of the objective lens.
In particular, at least one of the first and second relay lens groups may have a cemented lens which is composed of a plurality of lenses adhered with each other.
Further optionally, at least one of the first and second relay lens groups may consist of a telephoto type lens system.
Optionally, the optical system may include a cylindrical lens provided between the light source and the galvano mirror, the cylindrical lens converges the parallel beam on the reflection surface of the galvano mirror in a direction parallel to a rotary axis of the galvano mirror so that a line-like image is formed on the reflection surface of the galvano mirror, and the relay lens system is constituted such that a parallel beam is emerged from the relay lens system when the line-like image is reflected by the reflection surface of the galvano mirror and is incident on the relay lens system.
In particular, the relay lens system may include an anamorphic surface.
In this case, the relay lens system may include a first and a second relay lens groups, the first relay lens group has different power in a direction parallel to the rotary axis of the galvano mirror and in a direction perpendicular to the rotary axis of the galvano mirror, the first and second relay lens groups being arranged such that the beam reflected by the reflection surface of the galvano mirror is converged at a focal point of the second relay lens group, and that a rear focal point of the second relay lens group coincides with a front principal point of the objective lens.
According to another aspect of the invention, there is provided an optical system for an optical disk drive for reading/writing data from/in an optical disk, the optical system comprising: a light source emitting a diverging light from a light emitting point; a deflector having a rotatable mirror surface, the light emitted by the light source being incident on the mirror surface and is deflected; an objective lens system facing the optical disk; and a relay lens system provided between the deflector and the objective lens system such that the mirror surface and a front principal plane of the objective lens system have a substantially conjugate relationship.
In this case, the relay lens system may have a first and second relay lens groups which are arranged to satisfy relationship below:
1/f1=1/D1+1/D2;
and
f1 less than D1,
where, D1 is a distance between the light emitting point and a front principal point of the first lens group, fl is a focal length of the first lens group, and D2 is a distance between a rear principal point of the first lens group and a front focal point of the second lens group.